Download Absolute chronologies from the ocean: Records from the longest

Science Highlights: Open Section
4 Absolute chronologies from the ocean:
Records from the longest-lived, non-colonial animals on
Earth
Alan D. Wanamaker Jr., J.D. Scourse, C.A. Richardson, P.G. Butler, D.J. Reynolds and I. Ridgeway
School of Ocean Sciences, College of Natural Sciences, Bangor University, Wales, UK; [email protected]
Although there is an extensive network of
annually dated, terrestrial-based proxies in
the northern hemisphere for the last millennium (e.g., NRC, 2006), such records are
scarce in the marine realm. Most existing
annually resolved, marine-based proxy
records (corals) are biased to the tropical
oceans. In part, research in the mid- to
high-latitude oceans has been hindered by
a lack of suitable high-resolution marine
archives for sclerochronological studies.
Sclerochronology is the broad study concerning the accretionary hard tissues of
organisms (e.g., mollusks, corals, fish) and
can be regarded as the aquatic counterpart
of dendrochronology. Sclerochronological
techniques are used to develop a master
chronostratigraphy within biogenic carbonates, which is then used as a template
for growth and geochemical analysis.
The long-lived bivalve mollusk Arctica
islandica (Linnaeus, 1767), common in the
shelf seas of the temperate to sub-polar
North Atlantic, has enormous potential
as a high-resolution marine archive. This
stationary benthic clam is highly suitable
for environmental and climate studies
because (1) it is extremely long-lived (up
to 3-4 centuries; Schöne et al., 2005; Wanamaker et al., 2008a), (2) it produces annual increments in its shell (Jones, 1980;
Fig. 1), (3) regional increment series can
be cross-matched, demonstrating a common response to environmental forcing(s)
(Schöne et al., 2003; Fig. 2), (4) fossil shells
can be cross-matched and floating shell
chronologies can be constructed after radiocarbon dating (Scourse et al., 2006), (5)
live-caught shells can be cross-dated with
fossil shells to assemble very long, absolutely dated growth records (Marchitto
et al., 2000), (6) master shell chronologies
can be created that are as statistically robust as tree ring chronologies (Witbaard
et al., 1997; Helama et al., 2007), (7) Arctica islandica is widely distributed in the
mid- to high-latitudes of the North Atlantic
throughout the Holocene (see Scourse et
al., 2006), and (8) the geochemical signature (14C, δ18O, δ13C) from the shell material and master shell-growth chronologies
can be used to reconstruct ocean circulation, hydrographic changes, and ecosystem dynamics (Weidman and Jones,
1993; Weidman et al., 1994; Witbaard et
Figure 1: Idealized preparation of an Arctica islandica shell for sclerochronological studies (from Scourse et al.,
2006). A) External valve face, showing line of section through the left valve used to generate acetate peel replicas;
B) Acetate peel replica cross-section of valve showing annual growth band increments along shell margin (arrows)
and within the hinge plate. The axis of growth through the hinge plate used to generate increment data is indicated
by the dashed white line; C) Numbered annual growth bands are measured from the earliest growth bands to the
most recent along the axis of growth. Juvenile early bands are wide and reflect the ontogenetic growth curve of
the individual; D) Narrow senescent late bands from the outer part of the hinge plate axis.
al., 2003; Schöne et al., 2005; Wanamaker
et al., 2008b). The generation of annually
resolved marine proxy data from the extratropical Atlantic Ocean, using A. islandica
and other suitable archives (e.g., Halfar et
al., 2008) to document and interpret environmental change, will be a major research
goal for the next decade of marine paleoclimate research.
Some A. islandica research
highlights
Weidman and Jones (1993) first showed
the potential for using the radiocarbon
content from the annually banded shells
of A. islandica to monitor ocean circulation in the northwestern Atlantic (Georges
Bank). Interestingly, the oceanic response
to atmospheric-bomb testing during the
Figure 2: Incremental growth series measured from five dead-collected A. islandica shells from the Irish Sea (off
the Isle of Man) showing a high degree of synchronicity. Data are from P.G. Butler (unpublished). Each shell growth
record was normalized with a natural logarithmic function, then detrended with a 15-year spline to remove the
ontogenetic growth trend.
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Science Highlights: Open Section
1950s on Georges Bank was attenuated
compared to the tropical Atlantic. This result highlighted important differences in
ocean mixing processes between the lowand mid-latitudes, and it likely indicated
that there was a deepwater source for the
waters of Georges Bank. Further, it was illustrated that the oxygen isotopes from
the shell material of A. islandica were precipitated in isotopic equilibrium with the
ambient seawater (δ18Owater) (Weidman et
al., 1994). Therefore, sub-annual to annual
seawater temperature records could be
constructed if the δ18Owater, which is related
to salinity, could be constrained. It was
later shown that absolutely dated master
A. islandica shell chronologies could be
accurately constructed from live-caught
and fossil material (Marchitto et al., 2000).
Recently, it was demonstrated that fossil A. islandica shells could be successfully
cross-matched using initial ‘range finding’
radiocarbon measurements and a rigorous comparison among shell growth series
(Scourse et al., 2006). In the North Sea, shell
growth records from A. islandica were used
to infer changes in zooplankton cycles and
productivity, which seemed to be in part
related to the North Atlantic Oscillation
(NAO) (Witbaard et al., 2003). Schöne et al.
(2003) showed a remarkable positive relationship between the winter NAO index
and A. islandica shell growth series from
the central North Sea and the Norwegian
shelf, and suggested that the NAO was impacting shell growth via its influence on
food supply.
Recently, with ultra-high-resolution
sampling of an A. islandica shell, it was
demonstrated that there is no ontogenetic trend in oxygen or carbon isotopes
(Schöne et al., 2005). This result is important for reconstructing past ocean environments because it means that geochemical
records from both young and old shell portions can be used to infer past conditions.
In addition, it was found that A. islandica off
Iceland continue to deposit shell material
during the winter months, indicating that
there is no substantial ‘shut-down’ period
(Schöne et al., 2005). This result was later
corroborated in the Gulf of Maine (Wanamaker et al., 2008b). The data illustrated in
Figure 3 shows that A. islandica is comparable to corals, providing ultra-high-resolution proxy records for centuries at a time.
Recently, we counted 405 annual
bands in a sectioned A. islandica shell from
the north Icelandic shelf, and radiocarbon
analysis from the first annual shell layer
(ontogenetic age 0-1) confirmed its sclerochronological age (Wanamaker et al.,
2008a). We then calculated the reservoir
age of the waters north of Iceland for ca.
Figure 3: Oxygen isotope record (δ18O) from A. islandica shells from the Gulf of Maine (modified from Wanamaker
et al., 2008b); A) Modern relationship between a local sea surface temperature (SST) instrumental record and the
shell-derived water temperature at 30 m water depth. Nearby salinity values from 1928-2003 were used to constrain δ18Owater values, based on an established salinity/ δ18Owater mixing line; B) Late Holocene shell-derived water
temperatures (based on mean 50 m salinity values prior to 1928) and δ18O record (indicates a cooling trend over
the last millennium); C) Late Holocene shell-derived water salinity. Dotted line represents possible salinity increase
in the Gulf of Maine if long-term mean annual seawater temperatures remained constant. The data in both (B) and
(C) estimate maximum end-member conditions for the Gulf of Maine (see Wanamaker et al., 2008b for details).
AD 1600, which was found to be 637 ±
35 14C years. This reservoir age compares
rather well with tephra-based ages from
sediment cores from the same location,
which are about 600 14C years at AD 1650
(e.g., Eiríksson et al., 2004). The >400 year
A. islandica is considered to be the oldest mollusk yet discovered and perhaps
the longest-lived non-colonial animal. We
have begun to build a 1000-year master
shell chronology from the north Icelandic
shelf to reconstruct paleoceanographic
changes in a region that is very sensitive
to climate change. Preliminary results indicate that shell growth from the north Icelandic shelf is highly synchronous among
samples from this region, and approx. 50%
of the interannual growth variation is related to seawater temperatures during the
summer (Wanamaker et al., 2008a).
PAGES News • Vol.16 • No 3 • August 2008
Future applications
Shell data from A. islandica can be used
to reconstruct meridional overturning
circulation changes in key regions in the
North Atlantic. Radiocarbon analyses from
A. islandica shells can provide a continuous calibration of the marine reservoir age
with an absolutely dated chronology (e.g.,
Wanamaker et al., 2008a). Furthermore,
data from master shell chronologies can
help characterize rates and/or the type of
change (step-like, monotonic, complex)
over key climate periods during the Holocene (e.g., 8.2 kyr event, Medieval Warm
Period/Little Ice Age transition, and 20th
century warming) in key oceanographic
settings. Ultra-high-resolution geochemical data can provide seasonal information
(amplitude and variability) on water temperature changes and seasonal stratifica-
5 Science Highlights: Open Section
tion dynamics for the shallow shelf seas in
the North Atlantic. Recent advances in geochemical techniques, including the “carbonate clumped isotope” method (Ghosh
et al., 2006; Came et al., 2007), provide
an excellent opportunity to reconstruct
past ocean temperatures independent of
δ18Owater or salinity using A. islandica and
other suitable archives. Although A. islandica is restricted to the North Atlantic, other
long-lived bivalves with annual banding, such as the geoduck clam (Panopea
abrupta) from the Pacific, can also serve as
reliable climate proxies (e.g., Strom et al.,
2004), extending the geographical range
in which ocean climate can be reconstructed using the methods described here.
master shell-growth records have the potential to greatly improve our understanding of key climate events/transitions, as
well as ecosystem changes in the North Atlantic throughout much of the Holocene.
Its great longevity, its fidelity as a proxy
record, and its abundance and wide geographical distribution make A. islandica
a key proxy archive for the North Atlantic
region.
Summary
Note
A. islandica is a remarkable, yet underutilized, marine archive. Geochemical and
Acknowledgements
We thank the participants who attended the
A. islandica workshop held at the Gregynog
Conference Center, Wales in March 2008. This
research is funded by European Climate of the
Last Millennium (http://geography.swan.ac.uk/
millennium/) (Project no. 017008).
Isotope data for Figure 3 is available from the
NOAA paleoclimate database www.ncdc.noaa.
gov/paleo/data.html
References
Marchitto, T.M., Jones, G.A., Goodfriend, G.A. and Weidman, C.R., 2000:
Precise temporal correlation of Holocene mollusk shells using
sclerochronology, Quaternary Research, 53: 236-246.
Schöne, B.R., Fiebig, J., Pfeiffer, M., Gleβ, R., Hickson, J., Johnson, A.,
Dreyer, W. and Oschmann, W., 2005: Climate records from a bivalve Methuselah (Arctica islandica, Mollusca; Iceland), Palaeogeography, Palaeclimatology, Palaeoecology, 228: 130-148.
Scourse, J., Richardson, C., A., Forsythe, G., Harris, I., Heinemeier, J.,
Fraser, N., Briffa, K. and Jones, P., 2006: First cross-matched floating chronology from the marine fossil record: data from growth
lines of the long-lived bivalve mollusc Arctica islandica, The Holocene, 16: 967-974.
Wanamaker, A.D., Jr., Kreutz, K.J., Schöne, B.R., Pettigrew, N., Borns,
H.W., Introne, D.S., Belknap, D., Maasch, K.A. and Feindel, S.,
2008b: Coupled North Atlantic slope water forcing on Gulf of
Maine temperatures over the past millennium, Climate Dynamics, doi:10.1007/s00382-007-0344-8.
Weidman, C.R., Jones, G.A. and Lohmann, K.C., 1994: The long-lived
mollusk Arctica islandica: a new paleoceanographic tool for
the reconstruction of bottom temperatures for the continental
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For full references please consult:
www.pages-igbp.org/products/newsletters/ref2008_3.html
On the abyssal circulation in the Atlantic basin at the Last
Glacial Maximum
Olivier Marchal and William Curry
Department of Geology and Geophysics; Woods Hole Oceanographic Institution, USA; [email protected]
Our understanding of oceanic variability
on timescales longer than the time span
of direct oceanographic measurements
(about a century for most common measurements) relies on our capability to interpret the marine sediment record. Sediment observations have reached the point
where hypotheses regarding oceanic conditions during specific time intervals of the
geological past can be tested. An interval
of preeminent interest is the Last Glacial
Maximum (LGM, ca. 20 kyr BP), when large
ice sheets occupied North America and
northern Europe, and global sea level was
reduced by more than 100 m. Much effort
has been devoted to estimating oceanic
conditions during the LGM, in particular
in the Atlantic basin. Hypotheses regarding the ocean circulation during the LGM
are particularly relevant, given the postulated role of ocean circulation in climate
change. Here we report on a test of the
null hypothesis that observations from
glacial sediments in the Atlantic basin are
consistent with the modern circulation.
A conventional view
6 Among the most common measurements
performed on glacial sediments are two
isotopic ratios of calcite shells of benthic
foraminifera (bottom-dwelling organisms):
the oxygen isotopic ratio 18r = 18O/16O and
the carbon isotopic ratio 13r = 13C/12C. Both
ratios are usually expressed as a relative
Figure 1: Location of the sediment cores considered in the compilation of benthic foraminiferal data for the Holocene
(black dots; 198 measurements; defined as 0-3 kyr BP) and LGM (open red circles; 18O measurements; 18-21 kyr BP).
Compilation includes data from both earlier syntheses and other sources. The benthic δ18O and δ13C measurements
were conducted exclusively on the benthic taxa Cibicidoides and Planulina. Both post-glacial and glacial values
are available at most core locations. Locations span the depth range 280–5105 m, with 50% (80%) of the cores
from depths shallower than 2500 m (3750 m). Figure modified from Marchal and Curry, 2008.
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